1. Technical Field
The invention relates to the art of gas metal arc welding and particularly to the welding of aluminum-based torque tubes used for automotive drivelines.
2. Discussion of the Prior Art
In consumable electrode arc welding processes, an electrode of indefinite length is continuously fed to a welding arc, established between the electrode and a workpiece, where it is melted by the intense heat of the arc and fuses with the workpiece. Generally, the metal deposited from the consumable electrode is shielded with an inert shielding gas, and thus the reference to the process as gas metal arc welding. Consumable electrode welding is not only substantially faster than nonconsumable electrode welding but is particularly adapted to the automatic welding of carbon and stainless steel where it has been predominantly used. The shielding gas provides a more easily ionized path than obtained in air, aiding smooth transfer of current and functioning to surround the arc and weld pool with an atmosphere that is nonreactive with the molten metal. For reasons of economy, argon and helium are the only gases in general use when welding aluminum. Of these two gases, argon is the most commonly used and promotes greater arc stability than helium.
A high current density is often employed to break up the molten metal of the electrode into very fine droplets by an increase in its vapor pressure, resulting in deeper weld penetration at a rate of about 200 drops per second. However, potential damage to some thin or nonferrous workpieces may result from use of such high currents.
Pulsing of the current supply to the arc has been developed along with improved shielding gases to lower the average current density to promote the welding of ferrous-based articles (see U.S. Pat. Nos. 4,273,988; 4,507,543; 4,628,181; and 4,749,841). The arc current is cyclically pulsed between a minimal value needed to maintain the arc and a maximum value which may be several hundred percent larger. The pulse frequency may range from several cycles per second up to several hundred cycles depending on conditions at the particular welding operation. Such welding is conducted by using a current having a waveform determined by four factors: a pulse current Ip, a base current I.sub.B, a pulse duration Tp, and a base duration T.sub.B. A high current will flow for a short time between the electrode and the workpiece to be welded while the average current I.sub.M is maintained at a low value. By flowing at high current for a short time, the arc is more stable and highly concentrated, and penetration is considerably deeper and the bead is relatively wider.
In the above patent disclosures, the shielding gases have included minor proportions of an oxidizing gas constituent (O.sub.2 or CO.sub.2) to improve gas ionization and thereby facilitate all-position welding of ferrous workpieces. When gas metal arc welding is used for aluminum workpieces, the prior art is pointedly assertive that oxidizing gases must be avoided due to the interference of oxides formed with the aluminum. It has been stated frequently in the literature: "such oxygen-bearing shielding gases cannot be used when welding aluminum, as the production of refractory oxides inhibits proper metal transfer and deposition", taken from an article published in The Welding Journal, American Welding Society, pages 21-27, 1985, entitled "GMA Welding of Aluminum With Argon/Freon Shielding Gas Mixtures". Further documents setting forth this state of art include: (a) excerpts from handbook published by Kaiser Aluminum Company, 1967, pages 7-9 through 7-11; (b) welding handbook of American Welding Society, Volume 4, 7th Edition, 1978, Chapter 8, entitled "Aluminum Alloys", pages 347-348; (c) technical paper presented at Aluminum Association Meeting, dated March, 1988, entitled "MIG Welding of Aluminum", pages 6.15 and 6.28.
The use of aluminum and aluminum alloys in automobiles is becoming increasingly popular. Such popularity is mainly due to the fact that aluminum components can be manufactured much lighter in weight than comparable steel components which they replace without sacrificing strength or durability. However, it is most difficult to securely join such aluminum driveshaft components using existing aluminum welding knowledge to achieve the kind of quality and productivity speeds achieved with welding of steel. The prior art has resorted to multiple pass welding at low current levels with extraordinarily large welding deposits to achieve welds on aluminum driveshafts (see U.S. Pat. No. 4,542,280). The disadvantage of such technique is that weld speeds obtainable are not compatible with the high volume requirements of automotive welding. These low current levels increase process variance by the cumulative effect of adding joint gap variations to other process variations and dictate the use of small diameter wires and their inherent feeding problems. The elimination of joint gap requirements is extremely important in high volume welding situations.
Therefore, it is an object of this invention to provide a method of welding aluminum torque tubes at speeds twice the level of that capable by the existing prior art for aluminum MIG welding and with a weld quality significantly improved with respect to fusion penetration, fusion width, and lack of porosity.